OMG! I just had an amazing idea! The only weakness of these to builders is for a builder to cover the shooting hole with stone, so why don't we place a single trap door in front of the turtle! That way when enemy builders come over they wont be able to place a block there! (Genius)

Here's what I'd suggest as a little change to the last one posted
The gold is supposed to be where you leave the dirt, basically you have a skeletal frame as the original builder.
This is rather cheap, as posted above not very sturdy or very safe (from bombs/etc). You can make it safer from digging by adding more stone to the sides, but this doesn't particularly protect against bombs, this does however mean that if you plan to leave this as the bottom of a tower and build on it later, it's a very simple thing to do, and if this area becomes a liability you can stone it off at that point.

The way to cure against bombs being thrown IN at this point would be simple, digging a pit underneath, don't bother to put anything in the pit, just dig it.
This is just enough where you won't take (much) damage from the bomb as an archer up above, won't need to worry about falling (unless you're a scrub or something, then add a last bridge to the left side there).

As for sealing/moving up, that's also not too hard.

some stupid variations I made in a few seconds, I'll probably test these if I'm bored enough later:

To hurt those that bomb in from the right side there, impairs a bit of the bomb catchability of the bottom, but instead turns it into a short term hiding area for the archers from the knights, and allows them to shoot up at them as well.

A quick and dirty variation to make it mean nothing to attack the bottom relatively

I have a 95% belief that I can't be bothered testing that,a) The friendly team will be damaged by the spikes on team doors,andb)Team doors can't be placed on a workshop. At least, without editor they can't. c: